Image sensor structure and fabricating method therefor

ABSTRACT

An image sensor structure and a method for making the image sensor structure, for avoiding or mitigating lens shading effect. The image sensor structure includes a substrate, a sensor array disposed at the surface of the substrate, a dielectric layer covering the sensor array, wherein the dielectric layer includes a top surface having a dishing structure, an under layer filled into the dishing structure and having a refraction index greater than that of the dielectric layer, a filter array disposed on the under layer corresponding to the sensor array, and a microlens array disposed above the filter array. A top layer may be additionally disposed to cover the filter array and the microlens array is disposed on the top layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image sensor structure and a methodof fabricating the same, and particularly to an image sensor structureand a method of fabricating the same capable of avoiding or mitigatingshade effect.

2. Description of the Prior Art

For digital imaging apparatuses, such as digital still cameras ordigital video cameras, image quality is one of the most significantdesign issues. In an image generated by an image sensor of aconventional digital imaging apparatus, the central portion of the imageis typically brighter than the peripheral portion of the image. Thisphenomenon is also referred to as lens shading effect or vignettingphenomenon, which is caused by a non-uniform photo-response across thelens of the digital imaging apparatus. It is found that if the chief rayangle is greater than 20 degrees, the brightness of the peripheral imageis 78% or less of the brightness of the central image. Hence, variouslens shading compensation methods have been disclosed in order tomitigate the lens shading effect.

FIG. 1 illustrates a cross-sectional diagram of a conventional CMOSimage sensor structure. In the CMOS image sensor structure 10,photodiodes 12 are formed in the surface of a substrate 14. Apassivation layer 22 covers each of three metal conductor layers 16, 18,and 20. A dielectric layer 24 covers each passivation layer 22 and isplanarized. An under layer 26, which is planar and with a substantiallyuniform thickness, is formed on the up most dielectric layer 24. Colorfilters 28, such as red filters 30, green filters 32, and blue filters34 are formed on the under layer 26. A top layer 27 is formed on thecolor filters (CFs) 28. Microlenses 36 are formed on the top layer 27.With such structure, focal length can be sufficient for allowing lightto focus on the photodiodes 12 within the substrate 14, and,accordingly, the shading effect on such structure is not obvious.

However, with the demand for lighter, thinner, shorter, and smallerdevices, the focal length must be shortened. Thus, the chief-ray angleis increased and the shading effect becomes significant. FIG. 2illustrates a combination of a conventional CMOS image sensor structureand a lens module. Photodiodes 12 are located within the surface 15 ofthe substrate 14. All of the photodiodes, dielectric layers, orprotection layers are not shown for purpose of concise and easy reading.As shown in FIG. 2, in the situation that the focal length is too short,after the incident light 38 passes through the lens module 40, a lightbeam reaches the edge, passes through the microlens 42 or 43 at theedge, and passes through the top layer 27, the color filter (CF) 28, theunder layer 26 and the dielectric layer 46 to focus on the point A orA′; and another light beam reaches the center, passes through themicrolens 44 at the center, and passes through the top layer 27, thecolor filter 28, the under layer 26 and the dielectric layer 46 to focuson the point B. It can be found that the points B and A or A′ are not ona same planar face. The B point is lower. The arc line 48 roughlydepicts a line formed by all focus positions. In a plan view, the focuspositions form a concave face having a depth difference d from thehorizontal surface 15 of the substrate 14 where the photodiodes 12 arelocated, leading to non-uniform photo-response and making the peripheralimage darker.

FIG. 3 illustrates a conventional way to mitigate shading effect byshifting the microlens inward and/or shifting the color filter outward,in order to make the focuses to be along with the positions of thephotodiodes at the surface of the substrate to reduce the depthdifference, that is, to make the depth d′ (for point B′) to become zeroif possible. However, as shorter focal length is demanded, the chief-rayangle becomes greater, where the shifting of the microlenses or thecolor filters is spatially limited and can not well compensate theshading effect.

Therefore, there is still a need for a novel image sensor structurewhich is small, light, short, and thin, without suffering from shadingeffect.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide an image sensorstructure, in which shading effect can be avoided or mitigated, giving auniform photo-response.

The image sensor structure according to the present invention includes asubstrate; a sensor array disposed at the surface of the substrate; adielectric layer covering the sensor array, wherein the dielectric layercomprises a top surface comprising a dishing structure; an under layerfilled into the dishing structure and having a refraction index greaterthan that of the dielectric layer; a filter array disposed on the underlayer corresponding to the sensor array; and a microlens array disposedon the filter array.

In another aspect of the present invention, the method of fabricating animage sensor structure according to the present invention comprisessteps as follows. First, a substrate is provided. A sensor array isformed at the surface of the substrate. Thereafter, a dielectric layeris formed to cover the sensor array and the substrate. The top surfaceof the dielectric layer is formed into a dishing structure. Thereafter,an under layer is formed in the dishing structure, wherein the underlayer has a refraction index greater than that of the dielectric layer.A filter array is formed on the under layer. A microlens array is formedon the filter array.

In the image sensor structure according to the present invention, anunder layer is disposed beneath a filter array. The under layer isfilled into the dishing-shaped top surface structure of the underlyingdielectric layer. Material for the under layer is selected, such thatthe under layer can play as a role of a convex lens to compensate thefocal length difference between the center and the edge of the sensorarray, in addition to functions of adhesion and planarization. Hence,the photo-response in one chip will be more uniform.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional diagram of a conventional CMOSimage sensor structure;

FIG. 2 illustrates a combination of a conventional CMOS image sensorstructure and a lens module;

FIG. 3 illustrates a conventional way to mitigate shading effect;

FIG. 4 illustrates a cross-sectional diagram of an embodiment of theimage sensor structure according to the present invention;

FIG. 5 illustrates a combination of the image sensor structure accordingto the present invention and a lens module;

FIG. 6 illustrates a cross-sectional diagram of another embodiment ofthe image sensor structure according to the present invention; and

FIGS. 7-10 illustrate the method of fabricating an image sensorstructure according to the present invention.

DETAILED DESCRIPTION

The image sensor structure according to the present invention may beapplied to CMOS image sensor (CIS) devices or charge coupled devices(CCD) FIG. 4 illustrates a cross-sectional diagram of an embodiment ofthe image sensor structure according to the present invention. As shownin FIG. 4, the image sensor structure 50 includes a substrate 52, asensor array 54, a dielectric layer 56, an under layer 60, a filterarray 62, and a microlens array 64. The substrate 52 may be for examplea semiconductor substrate. The sensor array 54, i.e. an array of sensingelements, is disposed at the surface of the substrate 52. It may bedisposed in or on the substrate surface without particular limitationbut in accordance with the properties of the sensing element or devicedesign. The sensing element may be, for example, photo-sensing element,such as photodiode. The dielectric layer 56 covers the sensor array 54and the substrate 52. The dielectric layer 56 includes a top surface 58and is concave. Hence, the top surface 58 is in a shape of dish orshallow dish, forming a dishing structure. There may be a plurality oflayers of metal interconnects disposed within the dielectric layer 56for electric conduction or light shielding.

The dishing structure is filled with an under layer material to form anunder layer 60. The under layer 60 has a refraction index which may begreater, and preferably slightly greater, than the refraction index ofthe dielectric layer 56 and may be less than the refraction index of themicrolens. For example, the refraction index of the under layer 60 maybe within a range from about 1.5 to about 1.6, but not limited thereto.It may depend on optical properties as a whole. The under layer mayinclude a material of, in addition to the required refraction index,high transmittance and may also serve as a glue layer to combine thefilters and the dielectric layer 56 together. Furthermore, the underlayer may also have a planarization function for providing a planarsurface for the filters to be formed on. Furthermore, the under layermaterial may be preferably suitable for a filling process, forconvenient fabrication It may be selected from conventional materialsfor color filter top layer or under layer, but not limited thereto. Itmaybe, for example, a polymer, such as acrylic polymer, but not limitedthereto. The filter array 62 is disposed on the under layer 60 at alocation corresponding to the sensor array 54. The filter array 62 is anarray of a plurality of filters. The filters may be for example colorfilters or non-color filters, depending on the desire for the productsThe microlens array 64 is correspondingly disposed above the filterarray 62.

The depth of the dishing structure may depend on optical properties oras desired. The under layer has properties similar to a convex lens, andaccordingly the depth of the dishing structure may be properly decidedaccording to the size of the sensing region of the image sensorstructure, optical design, or optical properties of the under layer,microlens, filters, and the like. Integrally, with respect to a digitalimaging device, the depth of the dishing structure and focus number areallowed to be coordinated each other such that the light beams can focusalong the planar face of the sensor array to reduce the difference ofphoto-response from sensing elements.

Furthermore, the image sensor structure according to the presentinvention may further comprise a shielding layer for shielding light.The shielding layer is disposed in the dielectric layer to surround thedishing structure. The shielding layer may comprise a metal material,such as Ti, TiN, and the like. The shielding layer may be more rigidthan the dielectric layer such that, in the fabricating process of theimage sensor structure, the top surface of the dielectric layer is in adishing shape after performing a CMP process having selectivity betweenthe shielding layer and the dielectric layer. The removal of theperipheral portion of the dielectric layer in the sensing region isrelatively little due to the support by the shielding layer. The removalof the central portion of the dielectric layer in the sensing region iseasy and deeper due to the significant dishing effect for the CMPprocess, since it is the center of a large dielectric layer. Theshielding layer may comprise at least a ring structure. When theshielding layer comprises a plurality of layers of ring structure, thedistribution density of the shielding layer may gradually decrease (orin gradient) from the outer to the inner portion of the shielding layer.Alternatively, the shielding layer may comprise a plurality ofdiscontinuous segments, and the distribution density may also graduallydecrease (or in gradient) from the outer to the inner.

Furthermore, the image sensor structure may further comprise a top layer66 disposed on and enveloping the filters. The top layer may include thesame material as the under layer and allow a planar face to be formedabove the filter array.

FIG. 5 illustrates a combination of the image sensor structure accordingto the present invention and a lens module. As shown in FIG. 5, afterthe incident light 38 passes through the lens module 40, a light beamreaches the edge, passes through the microlens 64 a or 64 b at the edge,and passes through the top layer 66, the filter 62, the under layer 60,and the dielectric layer 56 to focus on the point C or C′; and anotherlight beam reaches the center, passes through the microlens 64 c at thecenter, and passes through the top layer 66, the filter 62, the underlayer 60, and the dielectric layer 56 to focus on the point E. It can befound that the points E and C or C′ are along the horizontal surface 55of the substrate 52 where the sensor array 54 are located. As a result,with respect to photodiode as a sensing element, under irradiation of asame light amount, substantially same extent of photoelectric conversioncan be obtained even for the photodiodes located at different positions.Accordingly, the central image and the peripheral image exhibit uniformbrightness.

In the present invention, in addition to fill the under layer materialinto the dishing structure to alter the optical refraction path,conventional methods for adjusting focal length may be further adopted.For example, as shown in FIG. 6, in an image sensor structure 70, aportion of the filters 72 are shifted outward, or a portion of themicrolenses 74 are shifted inward. That is, the edge portion of thefilter array and the central portion of the filter array have differentpitches for arrangement, or the edge portion of the microlens array andthe central portion of the microlens array have different pitches forarrangement. In addition, it may make the microlens positioned in theedge portion of the microlens array to have a shape different from themicrolens positioned in the central portion of the microlens array.These methods also have a function for further adjusting focal lengthand enhancing the focus to be located on the sensing element.

The image sensor structure according to the present invention may beobtained from the method as described below. As shown in FIG. 7, first,a substrate 52 is provided. A sensor array 54 is formed at the surfaceof the substrate 52. Thereafter, a dielectric layer 56 is formed tocover the sensor array 54 and the substrate 52. For example, thedielectric layer 56 may be formed by a chemical vapor depositionprocess. A plurality of metal layers may be further formed in thedielectric layer 56 and may be formed by, for example, a metalinterconnect process. A passivation layer comprising silicon nitride,silicon oxide, or the like may be formed by deposition to cover themetal layers.

Thereafter, the top surface 58 of the dielectric layer 56 is formed intoa dishing structure, which may be achieved by performing a CMP processto polishing the dielectric layer 56. For example, if the dielectriclayer is an oxide, an oxide CMP process is employed. The formulation andthe conditions for the CMP process maybe optimally adjusted as desired.For example, the polishing time period can be increased when it iswanted to increase the dishing extent. A shielding layer 80 may beadditionally formed in the dielectric layer 56 surrounding the dishingstructure. The shielding layer may reinforce the peripheral portion ofthe dielectric layer and retard the polishing rate in the CMP process.FIG. 8 shows a plan view illustrating that the shielding layer 80surrounds the sensor array 54. The outmost portion 82 is a scribing linefor cutting the completed image sensor structure into a separated chip.Additionally, a polishing stop layer may be further formed on thedielectric layer 56 surrounding the dishing structure before the CMPprocess. The polishing stop layer may prevent the portion of thedielectric layer above the shielding layer 80 from being removed toexpose the shielding layer 80. If the shielding layer 80 is a metal andexposed, it easily pollutes the elements or devices.

Furthermore, forming the top surface of the dielectric layer into thedishing structure may be carried out by the combination of a CMP processand an etching process. In detailed, after the top surface 58 of thedielectric layer 56 is formed into a dishing shape using a CMP process,the dishing-shaped surface of the dielectric layer 56 may be etchedusing an etching process. Since the etching rate and the etching timeare easily controlled, the depth of the dishing structure thus formedmay be controlled conveniently and precisely.

As shown in FIG. 9, after the dishing structure is formed, it is filledwith an under layer material to form the under layer 60. For example, apolymer material using PGMEA (propyleneglycol methyletheracetate) andEEP (ethyl 3-ethoxypropionate) as solvent is spin-coated in the dishingstructure and dried to form the under layer 60, which has a lighttransmittance of about 95%. It is not limited to fill up the dishingstructure with the under layer material and the filling level may dependon requirement or desire. In the embodiment shown in FIG. 9, the underlayer 60 is filled up and over the dishing structure to cover theoriginal surface of the dielectric layer 56.

Thereafter, as shown in FIG. 10, a filter array 62 is formed on theunder layer 60. A top layer 66 may be further formed on the filter array62. The material for the top layer 66 may be the same as the under layermaterial. It may be also formed using a coating method. Thereafter, amicrolens array 64 is formed on the filter array 62. Each filter iscorresponding to a microlens and a sensing element.

In comparison with conventional techniques, the method of the presentinvention is characterized in that a CMP process is performed on thedielectric layer to form a dishing structure and an under layer materialis filled into the dishing structure to form an under layer. The underlayer may advantageously have all of the functions of focal lengthadjustment, planarization, and filter adhesion. The under layer materialmay be selected from conventional materials used to make the under layeror the top layer for the filters (but not limited thereto). Furthermore,as planarization process is also required to form the dielectric layerin the conventional technique, the desired dishing structure can be alsoobtained by utilizing such CMP planarization in the present invention.Hence, it is convenient and will not increase complicated steps orcostly material to the novel fabrication process.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention.

What is claimed is:
 1. An image sensor structure comprising: asubstrate; a sensor array disposed at the surface of the substrate; adielectric layer covering the sensor array, wherein the dielectric layercomprises a top surface comprising a dishing structure; an under layerfilled into the dishing structure and having a refraction index greaterthan that of the dielectric layer; a filter array disposed on the underlayer corresponding to the sensor array; and a microlens array disposedon the filter array.
 2. The image sensor structure of claim 1, whereinthe sensor array is a photosensor array.
 3. The image sensor structureof claim 1, wherein the refraction index of the under layer is from 1.5to 1.6.
 4. The image sensor structure of claim 1, wherein the refractionindex of the under layer is less than that of the microlenses.
 5. Theimage sensor structure of claim 1, wherein the under layer is a gluelayer and has a flat top surface.
 6. The image sensor structure of claim1, further comprising a shielding layer disposed in the dielectric layerand surrounding the dishing structure.
 7. The image sensor structure ofclaim 6, wherein the shielding layer comprises a metal material.
 8. Theimage sensor structure of claim 6, wherein the shielding layer comprisesat least a ring structure.
 9. The image sensor structure of claim 8,wherein the shielding layer comprises a plurality of layers of ringstructure, and the distribution density of the shielding layer graduallydecreases from the outer portion of the ring structure to the innerportion of the ring structure.
 10. The image sensor structure of claim6, wherein the shielding layer comprises a plurality of segments. 11.The image sensor structure of claim 10, wherein the distribution densityof the shielding layer gradually decreases from the outer to the inner.12. The image sensor structure of claim 1, wherein the microlenspositioned in the edge portion of the microlens array is in a shapedifferent from the microlens positioned in the central portion of themicrolens array.
 13. The image sensor structure of claim 1, wherein theedge portion of the microlens array and the central portion of themicrolens array have different pitches.
 14. The image sensor structureof claim 1, wherein the edge portion of the filter array and the centralportion of the filter array have different pitches.
 15. The image sensorstructure of claim 1, further comprising a top layer disposed on thefilter array and enveloping the filters, wherein the top layer comprisesa material the same as the under layer.
 16. A method of fabricating animage sensor structure, comprising: providing a substrate; forming asensor array at the surface of the substrate; forming a dielectric layercovering the sensor array and the substrate; forming the top surface ofthe dielectric layer into a dishing structure; forming an under layer inthe dishing structure, wherein the under layer has a refraction indexgreater than that of the dielectric layer; forming a filter array on theunder layer; and forming a microlens array on the filter array.
 17. Themethod of claim 16, further forming a shielding layer in the dielectriclayer surrounding the dishing structure.
 18. The method of claim 16,wherein, forming the top surface of the dielectric layer into thedishing structure is carried out through planarizing the dielectriclayer using a chemical mechanical planarization process.
 19. The methodof claim 18, before the chemical mechanical planarization process,further comprising: forming a polishing stop layer on the dielectriclayer surrounding the dishing structure.
 20. The method of claim 16,wherein, forming the top surface of the dielectric layer into thedishing structure is carried out by steps of: planarizing the dielectriclayer to form a dishing shape using a chemical mechanical planarizationprocess; and performing an etch process on the dielectric layer forcontrolling the depth of the dishing shape to form the dishingstructure.
 21. The method of claim 16, further forming a top layer onthe filter array to envelop the filters, wherein the top layer comprisesa material the same as the under layer.
 22. The method of claim 16,wherein the sensor array is a photosensor array and the photosensorarray is corresponding to the filter array.